Cells are isolated from the outside by the cell membrane, which is impermeable to hydrophilic physiologically active substances such as proteins and nucleic acids. Absence of methods for delivering such hydrophilic physiologically active substances into the cell has prevented clinical use of many hydrophilic physiologically active substances whose sites of action are located in the cells.
Several techniques to deliver such cell-impermeable hydrophilic physiologically active substances have been reported. The most popular method is one which gives hydrophobicity to a hydrophilic physiologically active substance using a lipid, and, by this, permeability through the cell membrane can be increased. Further, a technique wherein a peptide ligand is used to increase the permeability has also been reported.
A peptide ligand having a property which allows its transfer from the outside of the cell into the cell without destroying the cell membrane is called a cell-penetrating peptide. Examples of well-known cell-penetrating peptides include oligoarginines, wherein arginines are linked to each other; Tat (Patent Document 1), which is derived from HIV-1 virus; and penetratin (Patent Document 2), which is a peptide derived from Drosophila. In addition to these, various cell-penetrating peptides, such as those characterized by simple basicity, those characterized by amphipathicity of the primary structure or the secondary structure of the peptide, and those having a mechanism which has not been clarified, have also been reported. In addition to the permeability of the peptide itself into the cell, its use for delivering into the cell a hydrophilic physiologically active substance such as a gene to which the peptides is linked as a vehicle has been extensively studied. However, although these peptides have been successful as reagents for research, only a small number of peptides can be used for clinical application. Therefore, the peptides have been studied in various ways, and, in each use, sequences with which the peptides can be more efficiently transferred into cells are being searched.
A part of cell-penetrating peptides are reported to be capable of promoting permeation of a hydrophilic physiologically active substance through the mucosal epithelial cell layer in cases where these are orally or intranasally administered together with the hydrophilic physiologically active substance, allowing the hydrophilic physiologically active substance to pass into the general circulation with a high efficiency. It is thought that not all of the cell-penetrating peptides have the permeation-promoting capacity through mucosa, and that not only cell membrane permeability but also the intracellular dynamics, separation from the cell, and the like are involved in the permeation-promoting capacity. Thus, it is thought that only a part of the cell-penetrating peptides, which satisfy these conditions, have the ability to promote transmucosal absorption of hydrophilic physiologically active substances. However, only a small number of types of peptides are reported to have such a function. Further, the peptides for which the transmucosal absorption-promoting capacity has been reported are those with which the effect can be confirmed only in cases where they are used in the forms of conjugates with particular physiologically active substances (Patent Documents 2 and 3); and those which require, even in cases where the possibility that the peptides can be used in the forms not requiring covalent bonding with a drug is shown, large amounts of the cell-penetrating peptides in order to obtain a sufficient effect (Patent Document 4). Therefore, many problems remain to be solved before practical use of the peptides.
In recent years, in addition to low molecular hydrophobic drugs, which have been mainly used so far, hydrophilic physiologically active substances are drawing attention as candidate compounds of pharmaceuticals. Although the hydrophilic physiologically active substances have shown remarkable therapeutic effects, their poor transmucosal absorbability has limited the method of their administration almost only to injection. Therefore, development of a technology to allow absorption of hydrophilic physiologically active substances through mucosa is strongly desired for the purpose of non-injection administration of hydrophilic physiologically active substances. In particular, the absorption-promoting technique using a cell-penetrating peptide is expected to show a lower level of stimulation to mucosa compared to the absorption-promoting technique using surfactants, which has been extensively studied so far, and discovery of cell-penetrating peptides that cause efficient promotion of absorption may lead to development of a promising technique.